1. Field of the Invention
The present invention relates to a camera provided with a focus detection unit, such as a single lens reflex camera, particularly to a camera having a function of preventing foreign substances from adhering to a field lens included in an optical system of the focus detection unit.
2. Description of the Related Art
Single lens reflex cameras are provided with a focus detection unit that detects a focus state of an image taking optical system. The focus detection unit has, as disclosed in Japanese Patent Laid-Open No. 59-107312, a focus detection optical system including a field lens that divides a light flux from the image taking optical system into two light fluxes, and a secondary image-forming lens that causes the two light fluxes to reform paired optical images (object images). The focus detection unit converts the paired object images into electric signals (image signals) by a light-receiving element, and detects the focus state of the image taking optical system on the basis of a phase difference of the paired image signals.
The field lens is disposed at a primary image-forming surface of the image taking optical system or at the vicinity thereof, and is exposed to an outside of the camera through a mount to which an interchangeable image taking optical system (interchangeable lens) is detachably attachable. Adhesion of foreign substances, which enter into a camera body through the mount, to a lens surface (entrance surface) of the field lens causes images of the foreign substances to overlap the object images on the light-receiving element. Such overlap of the foreign substances on the object images makes it impossible to acquire normal image signals, which deteriorates focus detection accuracy.
Japanese Patent No. 2757387 discloses a camera that relatively moves an optical element (optical filter) included in a focus detection optical system and a brush to remove foreign substances adhering on the optical element.
However, the configuration disclosed in Japanese Patent No. 2757387 in which the optical element and the brush are relatively moved to remove the foreign substances adhering on the optical element needs a space where the optical element or the brush is moved and a driving mechanism that drives the optical element or the brush is placed, which increases the size of the camera.